Modeling of Ring Current Enhancement and Decay During Magnetic Storms

Physics

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2708 Current Systems (2409), 2730 Magnetosphere--Inner, 2740 Magnetospheric Configuration And Dynamics, 2778 Ring Current, 2788 Storms And Substorms

Scientific paper

The processes of ring current development and decay in the Earth`s magnetosphere during magnetic storms are studied. We trace protons with 90 degrees pitch angles numerically in the drift approximation. Tracing is performed in stationary and time-dependent magnetic and electric field models. We use Tsyganenko T89 magnetic field model as a baseline model with dipolar field for the internal sources. We introduce a model for storm-time ring current magnetic field, where we use a bean-shaped current system with a cross-section close to the observed distribution of trapped particles in the inner magnetosphere. The model is symmetric both longitudinally and in the north-south direction. The current density in the radial direction varies as a Gaussian. The latitudinal distribution of the current density is specified by an "anisotropy index", which increase gives particle distribution concentrated closer to the equator. As stationary electric field we use Kp-dependent Volland-Stern large scale electric field model with corotation electric field. Time-dependent electric field is given by Gaussian electric field pulse. The time-dependent magnetic field from the pulse is obtained from Faraday`s law. The initial distribution is Maxwellian with average temperature of 5 keV for particles with energies of 0.05-50 keV at 8-25 Re, 1900-0500 MLT. We account for losses associated with charge-exchange processes and Coulomb scattering. One storm is studied in detail: On My 2, 1998 Dst reached -85 nT. The ring current in the Tsyganenko T89 magnetic field model is replaced by our new ring current representation. The model free parameters are specified using observations from GOES and Polar satellites and Dst measurements for each time step separately. We model the substorm activity by several electric field pulses at times of the substorm onsets. The role of inductive electric fields and particle injections associated with substorms in the storm time ring current development is dicussed.

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